Familial aggregation of major depressive disorder has been well established, but identification of specific genetic markers underlying this condition has been challenging. The aims of this chapter are to: (1) provide an overview of the results of twin, family, and adoption studies, which have demonstrated that both genetic and environmental factors contribute to the aetiology of depression; (2) discuss studies of depression in the molecular genetics era, including linkage, association, genome-wide association, sequencing, polygenic, and pharmacogenetic studies; and (3) review challenges in identifying genetic factors underlying depression. The next phase of research in the genetic epidemiology of depression will require integration of research on genetic and environmental risk factors and increased recognition of the role of biologic and genetic vulnerability factors for depression.